Accelerometer and method of manufacture



Jan. 11, 1966 D. P. FAULK ETAL 3,229,128

ACCELEROMETER AND METHOD OF MANUFACTURE Filed Oct. 16, 1962 4Sheets-Sheet 1 INVENTORS DON/MD REM/4K BY J4m5 52077 1 D. P. FAULK ETALJan. 11, 1966 4 Sheets-Sheet 5 Filed Oct. 16, 1962 Jan. 11, 1966 D. P.FAULK ETAL 3,229,128

ACCELEROMETER AND METHOD OF MANUFACTURE Filed Oct. 16, 1962 4Sheets-Sheet 4.

F IIII lllll] INVENTORS DONALD PEu/LK .l 1 LII/ 155 Skoymmmy UnitedStates Patent 3,229,128 ACCELEROMETER AND METHOD OF MANUFACTURE DonaldP. Faulk, Fullerton, and James S. Gassaway, West Los Angeles, Calif.,assignors to Electra Scientific Corporation, Fullerton, Calif., acorporation of California Filed Oct. 16, 1962, Ser. No. 230,962 15Claims. (Cl. 3108.4)

This invention relates to accelerometers and more specifically concernsa novel and improved accelerometer and method of manufacture.

The manufacture of accelerometers has presented numerous difficultiessince the device must be highly sensitive to changes in accelerationalong a selected axis and at the same time have a minimum response toacceleration in a plane normal to the selected axis. While the majorelement or transducer forming part of an accelerometer may take anydesired form, in most instances the transducer utilizes a mass togetherwith means for measuring displacement of that mass as a result ofacceleration.

One form of transducer that has been found effective for the measurementof acceleration includes a central supporting member having crystalscemented to opposing faces and relatively heavy masses cemented to theouter faces of the crystals. When the transducer is accelerated, themasses tend to resist displacement, and such resistance stresses thecrystals and produces a voltage proportional to acceleration.

Notwithstanding the nature of the particular transducer that may beemployed, it has been found that manufacturing tolerances are suflicientto produce materal inaccuracies in a resultant transducernotwithstanding the care and precision utilized in mounting thetransducing element within a supporting housing. As a result, it hasbeen the practice to manufacture material quantities of accelerometerswith the axis of the transducer accurately aligned with the housing, andthen by actual tests of the completed units, sort them in accordancewith their measured accuracies. These manufacturing techniques haveindicated that only a few percent of the total number of transducersmanufactured will have a cross axis response of the order of /2 to 1% ofthe response in the desired axis of motion.

United States Patent Application Serial No. 194,937, filed May 15, 1962,entitled Accelerometer discloses an improved accelerometer wherein thetransducer is mounted within the supporting housing in a manner thatwill enable the manufacture of precision transducers on a productionbasis wherein the cross axis response can be maintained as low as /2 to1% of the response in the desired axis of motion.

This invention constitutes an improvement of the above mentionedapplication for patent and provides a novel and improved accelerometerand method of manufacturing that greatly facilitates the alignment ofthe transducer within the housing. With this procedure, it is possibleto determine the accuracy of a completed unit during the course ofmanufacture and reject defectivetransducers before completing the unit,thereby effecting material saving in manufacturing costs and insuringuniformly high quality and precision of all completed units.

Another object of the invention resides in the provision of a novel andimproved method for manufacturing accelerometers that greatlyfacilitates precise positioning of the accelerometer transducer within aprotective housing.

Still another object of the invention resides in the provision of anovel and improved accelerometer.

The above and other objects and advantages of this invention will becomemore apparent from the following description and accompanying drawingsforming part of this application.

In the drawings:

FIGURE 1 is a perspective view of a transducer and mount assembly inaccordance with the invention and preparator to installation in ahousing.

FIGURE 2 is an enlarged side elevational view of the structure shown inFIGURE 1.

FIGURE 3 is a plan view of the structure shown in FIGURE 2.

FIGURE 4 is a side elevational view of the structure shown in FIGURE 1mounted within a housing in accordance with the invention andillustrating one step in the procedure for adjustably postioning thetransducer relative to the housing.

FIGURES 5 and 6 are side elevational views in partial section showingsuccessive steps in securing the transducer within the housing.

FIGURE 7 is a side elevational view of the complete accelerometer inaccordance with the invention and with portions broken away to show theconstruction thereof.

FIGURE 8 is a cross-sectional view of FIGURE 7 taken along the line 88thereof.

FIGURE 9 is a perspective view of a complete accelerometer atapproximately twice normal size.

FIGURE 10 is a perspective view of an accelerometer similar to thatshown in FIGURE 9 but utilizing a modified housing.

As pointed out in the aforementioned application, for United StatesPatent, of which this invention constitutes an improvement, certain ofthe difficulties heretofore encountered in the manufacture ofaccelerometers stem from the characteristics of the crystals and themanufacturing techniques. All crystals including polarizedpolycrystalline ceramics, have electromechanically sensitive directionsof excitation. In the case of natural materials or single crystalmaterials, the desired mode of operation is selected by cutting inproper relat-on to the natural axis of piezoelectric orientation. In thecase of polycrystalline ceramics, the desired mode of operation may beachieved by polarizing and electroding in selected directions. In theexample shown in FIG. 1, the shear mode is used. The elements respond,therefore, to shear stresses as shown in FIG. 1. The electrical outputis a maximum when the mechanical force is applied parallel to thesensitive axis and decreases sinusoid-ally to zero as the direction offorce application is rotated until it is normal to the sensitive axis.

In the use of crystals for seismic transducers to measure acceleration,it is evident that the transducer will generate a voltage underconditions wherein the stress is applied at an angle other than to thestress or electromechanical axis. Under ideal conditions, stressesapplied at precisely 90 to the electromechanical axis will produce anegligible signal. Thus, known manufacturing techniques have emphasizedthe importance of maintaining extremely close tolerances of the crystalsand the mounts in order to attain precise alignment of theelectromechanical axis of the crystal with the axis of symmetry of thetransducer. Furthermore, balanced crystal arrangements utilizingmultiple crystals have afforded substantial improvement, since theutilization of two crystals balanced one against the other cancelledsome of the inaccuracies and provided improved mechanical and electricalsymmetry. Nevertheless, the need for extremely close manufacturingtolerances greatly affected the cost of the completed structure and evenaccuracies of the order of 1% could not be obtained except by selectionfrom a large number of units.

This invention provides not only an improved manufacturing procedure forthe adjustment of the transducing element Within the surroundinghousing, but also means for securing the transducer permanently inposition and at the same time preventing displacement of the transducerrelative to the housing once it has been secured therein.

The transducer and its mounting means or base in accordance with theinvention is generally denoted by the numeral 10. The taransducerincludes a rectangular central support 11 of substantial thickness, anda pair of crystals 1'2 and 13 cemented to opposite faces of the centralsupport 11. Relatively heavy masses 14 and 15 are cemented to the outerfaces of the crystals 12 and 13 respectively. The crystals 12 and 13 maybe formed of any suitable material that will generate a voltage asmeasured between the opposing faces thereof when subjected to shearingstresses.

One form of crystal that provides excellent results comprises apolarized ceramic element having metallic coatings on opposing facesthereof. With reference to the ceramic crystal 12, the metallic coatingsare applied to the faces 12a and 12b, while on crystal 13 the metalliccoatings are applied to surfaces 13a and 1312. While these metalliccoatings may be connected in any suitable manner to produce an outputvoltage, the coatings 12a and 13a are preferably connected together andcoatings 12b and 13b are preferably connected together. Theseconnections form the output terminals of the device and are connected tothe leads 16 and 17 as shown more clearly in FIG. 7. For purposes ofclarity, the actual connections have been omitted from the drawing.

The central support 11 is either secured to or formed integrally with acircular base 18 having a flat top surface 19 and a hemispherical bottomsurface 20. A shaft 21 having a threaded portion 22 extends downwardlyfrom the center of the base 18 and is preferably formed integrally withthe base 18.

As described in the above mentioned application for United StatesPatent, it was found that if the transducer is properly aligned withinthe surrounding housing or case, the response of the transducer to crossaxis motion can be maintained at a minimum value and compensate formanufacturing tolerances normally encountered in the fabrication of thetransducers themselves.

In accordance with this invention, a novel and impnoved method andstructure is employed to hold the transducer within the housing duringassembly and testing of the unit and then permanently secure thetransducer within the housing after it has been precisely aligned. Thesteps in the fabrication of an accelerometer in accordance with theinvention are shown in FIGS. 4, and 6.

The housing illustrated in the embodiment of the invention is generallydenoted by the numeral 23 and includes an essentially cylindrical wall24 and an end portion or wall 25. The end wall includes a centralopening 26, and the internal surface 27 is preferably provided with aspherical curvature corresponding to the curvature 20 of the base 18.The top of the housing 23 is provided with a tapered edge 28 to receivea top terminal as will be described.

Preparatory to the mounting of the subassembly within the housing 23,the surface 27 of the housing is coated with a cement 29 such as epoxyresin or the like, and the subassembly 10 is placed in position with theshaft 21 extending through the opening 26 in the housing 23. The unit isthen placed on a vibrating table 36) having a tapered opening 31therein. A spherical ball 32 having a central opening 33 is placed overthe shaft 21 and moved into engagement with the sides of the conicalopening. A helical spring 34 is then placed on the shaft 21, and thisassembly is held in place by a washer 35 and a nut 36, the latterengaging the threaded section 22 of the shaft 21. The nut 36 istightened until sufficient stress is placed on the shaft to hold thesubassembly 10 and the housing 23 in a fixed position on the table 30.The leads 16 and 17 from the transducer are then connected to suitablerecording means, and the table 30 is then vibrated first in onehorizontal direction and then in a second horizontal direction at rightangles to the first horizontal direction. During vibration in eachhorizontal direction, the angular position of the transducer It with inthe housing 23 is adjusted until a minimum signal is obtained on leads16 and 17. When these cross axis motions have minimum influence on thesignal produced on the leads 16 and 17, the transducer is properlyadjusted within the housing, and the unit is now ready to be completed.It has been found that in actual practice, the cross axis influence onthe accelerometer can be readily adjusted to within /2 to 1% ofequivalent motion along the axis of the housing 23. The central axis ofthe housing is known as the principal axis of the accelerometer and isdenoted herein by the numeral 37, while the axis of the transduceritself is denoted by the numeral 38. The magnitude of the angle betweenthe axes 37 and 33 will vary with each transducer as well as the angularposition of the plane defined by the two axes.

After the transducer has been positioned as shown in FIG. 4, the housingmay be heated to accelerate setting of the epoxy resin or if desired,the unit may remain in position for a period of twenty-four tothirty-six hours to permit the resin to set at room temperature. As soonas the resin has hardened, the transducer 10, together with the housing23, is removed from the vibrating table .30 by disengagement of the nut36 and removal of the washer 35, spring 34 and ball 32. The shaft 21 isthen cut off as close to the base 18 as possible and the remainingportion of the shaft may then be removed by an endmill and othersuitable means. The base 20 is then drilled and tapped as shown in FIG.5. In order to facilitate alignment of the drilled and tapped hole whichis denoted by the numeral 39 in FIG. 5, the housing 23 is provided withan annular bottom surface 40 defining a plane that is precisely at rightangles to the central axis 37 of the housing. Since the surface 40 andthe adjoining annular groove 41 are concentric with the axis 37, one orboth of these surfaces may be used to guide drilling and tappingapparatus in the formation of the threaded hole 39.

After the hole has been completed, a preformed stud 42 having a narrowshank 43 including a threaded portion 44, an annular flange 45 having anouter tapered edge 46 and a threaded section 47 is inserted into theopening 26 in the housing and with the threaded portion 47 engaging thethreaded opening 39. If desired, the stud 42 may be reduced in weight byproviding a central opening 48 extending therethrough. In normalpractice, it is preferable to use a suitable resin or cement on thethreaded stud 47 and the inclined surface 46 so that when the stud istightened in place, it will, in effect, form an integral part of thecompleted accelerometer. Since the surface 46 has the same inclinationas the opening 26 in the housing, these surfaces will contact to closethe opening 26 and at the same time cooperate with the stud 47 tosecurely hold the transducer in the adjusted position within thehousing.

The assembly of the accelerometer thus far described is completed byattachment of a top terminal assembly generally denoted by the numeral49 in FIG. 7. This terminal comprises an annular ring 50 which engagesthetop edge of the housing wall 24 and is held in place by cement orother suitable fastening means. The center:

portion of the terminal 49 comprises a hollow stud 51 having a threadedportion 52 and an annular flange 53. The flange 53 is secured in coaxialalignment with the ring 50 by the utilization of a suitable insulatingmaterial 34 that will adhere to the edges 50 and 53' of elements 56 and53 respectively to form a substantially unitary structure. The hollowstud'51 includes a central metallic sleeve secured in place by aninsulating material 56 filling the space between the sleeve 54 and theinner wall of the hollow stud 51 to a point spaced from the top surfaceof the stud 51. The lead 16 is inserted through the sleeve 55 and iselectrically connected thereto by the utilization of solder 57. The lead17 is inserted throu an opening 58 in the annular flange 53 and solderedto the flange at 59.

The foregoing arrangement provides a hermatically sealed unit with theelectrical connections being made by the attachment of a suitable maleconnector to the female connector 49. When the transducer is mounted inposition to measure acceleration of a vehicle, the accelerometer mountshould preferably include a flat surface normal to the direction ofmotion, and a threaded opening to receive the threaded mounting stud 42.It has been found that even if the opening to receive the stud 42 is notprecisely perpendicular to the mounting surface, the annular face 40 onthe bottom of the accelerometer will serve to precisely align theaccelerometer with the mounting surface. For convenience in mounting theaccelerometer, the bottom portion of the housing 23 is provided with ahexagonal configuration 23 to accommodate a suitable wrench. Thefinished accelerometer as described above is shown in FIG. 9 and it issubstantially twice the normal size of actual units fabricated by thismethod. It is understood, of course, that the same procedure may beutilized with units of any size and configuration. It is furtherunderstood that transducers other than the specific transducer describedherein may also be mounted in accordance with this invention.

FIG. 10 shows a transducer 60 substantially identical to the transducer24 except that the top of the housing 61 is closed by a flat cover orplate 62, and the side of the housing includes an opening 63 foraccommodation of a terminal assembly 49' substantially identical to theterminal assembly 49 illustrated in FIG. 7.

While only one embodiment of the invention has been illustrated anddescribed, it is evident that modifications, alterations and changes maybe made without departing from the true scope and spirit thereof asdefined in the appended claims.

What is claimed is:

1. In the manufacture of an accelerometer, the method of mounting atransducer carried by a base in a selected position within a housinghaving a closed end portion with an opening extending therethrough,wherein said housing has a principal mechanical axis and a mountingsurface on the outer side of said closed end portion which is normal tosaid axis, and said transducer has an electromechanical axis, comprisingthe steps of adjustably securing said transducer in said housing,subjecting the transducer and housing to velocity changes in the planeof said mounting surface while measuring signals produced by saidtransducer as a result of said velocity changes, adjusting saidtransducer relative to said mount to minimize the signals produced bysaid velocity changes, then with the transducer held in the adjustedposition forming a hole in said base with the axis of said hole normalto said mounting surface and substantially coincident with the axis ofsaid opening and then fixing a mounting stud in said hole and inengagement with said housing to hold said transducer in said adjustedposition.

2. In the manufacture of an accelerometer, the method of installing anacceleration sensitive transducer on a carrier member comprising thesteps of mounting said transducer on a base having a curved surface witha shaft extending from said surface, forming a curved surface on saidcarrier member with an opening extending therethrough, the last saidsurface having a contour for cooperatively receiving the first saidcurved surface with said shaft extending through said opening,positioning said transducer on said carrier and in a selected angularrelationship to said carrier member by means of said shaft, holding saidtransducer in said selected angular relationship independently of saidshaft, removing said shaft, and then mechanically securing said base tosaid carrier member.

3. In the manufacture of an accelerometer, the method according to claim2 including the step of forming a plane surface on said carrier member,and wherein said transducer is positioned relative to said planesurface.

4. In the manufacture of an accelerometer, the method of installing anacceleration sensitive transducer on a carrier member comprising thesteps of mounting said transducer on one side of a base, forming acurved surface on the other side of said base, said base including ashaft extending from said curved surface, forming a curved surface onone side of said carrier member, an accelerometer aligning surface onthe other side thereof and an opening extending therethrough, placing acement on at least one curved surface and then placing said curvedsurfaces in overlying relationship with said shaft extending throughsaid opening, adjusting said transducer relative to said aligningsurface while stressing said shaft to hold said curved surfaces in tightengagement, setting said cement to hold the carrier and base one to theother, severing said shaft from said base, drilling a hole in said basehaving its axis substantially coincident with the axis of the first saidopening and then securing a mounting stud in said hole.

5. In the manufacture of an accelerometer according to claim 4 whereinsaid stud is formed with carrier member engaging means and said engagingmeans is placed in pressure relationship to said carrier member.

6. In the manufacture of an accelerometer according to claim 4 includingthe steps of forming the first said opening with a tapered periphery,forming said hole with screw threads, forming said stud with threadedend portions and an intervening annular flange with a tapered perimeterand engaging one threaded end of said stud with said threaded hole andwith said tapered surfaces in pressure engagement one with the other.

7. In the manufacturer of an accelerometer according to claim 6 whereinsaid carrier member comprises a transducer enclosing housing.

8. An accelerometer comprising a housing, a closed end on said housing,an acceleration detecting transducer including a base within saidhousing, cooperating means on said base and inner surface of said closedend for holding said transducer in predetermined alignment With saidhousing, and mechanical means extending through said housing and securedto said base to hold said transducer in said predetermined alignment.

9. An accelerometer comprising a housing, a closed end on said housing,an acceleration detecting transducer including a base within saidhousing, cooperating means on said base and inner surface of said closedend for holding said transducer in predetermined alignment with saidhousing, and mechanical means extending through said housing and securedto said base to hold said transducer in said predetermined alignment,said mechanical means comprising a threaded stud having housing and baseengaging means to hold said cooperating surfaces in pressure engagementone with the other.

10. An accelerometer comprising a housing having a peripheral wall,means closing one end of said housing having an inner curved surface anda central opening therethrough axially aligned with said housing, atransducer base having a curved surface for cooperation with the firstcurved surface, a transducer within said housing and carried by saidbase, and a stud extending through said opening and engaging said baseto secure said base to said housing, said stud being axially alignedwith said housing.

11. An accelerometer according to claim 10 wherein said opening istapered outwardly, said base includes a threaded hole and said studincludes a threaded end portion engaging said threaded hole and atapered portion in pressure engagement with said tapered opening.

12. An accelerometer according to claim 11 wherein said curved surfacesare cemented one to the other.

13. An accelerometer according to claim 10 wherein said housing includesa flat surface surrounding said opening and defining a plane normal tothe axis of said housing and said st-ud is normal to said plane.

14. An accelerometer comprising a housing having a peripheral wall,means closing one end of said housing, said closing means having aninner concave surface, a tapered opening extending therethrough and anouter surface, said outer surface defining a plane normal to the axis ofsaid housing, a transducer assembly including a balanced, inertia type,ceramic crystal transducer carried by a base, said base having an outerconvex curvature corresponding in configuration With the first saidcurved surface, said transducer assembly being disposed within saidhousing with said curved surfaces cemented one to the other and withsaid transducer adjusted relative to said housing axis whereby saidtransducer produces a minimum signal as a result of cross-axisvibration, and a mounting stud threaded end portions and a taperedcenter portion, said base including a threaded opening coaxially alignedwith said housing and one threaded end portion of said stud vthreadablyengages said base with the References Cited by the Examiner UNITEDSTATES PATENTS 2,727,296 12/ 1955 Craiglow 29-2535 2,763,050 9/ 1956Ciccolella 2925.35 3,120,622 3/ 1960 Dranetz et al. 3 l0-8.4 3,093,7596/1963 Orlacchia 3108.4 3,104,335 9/1963 Shoor 310-8.4

ORIS L. RADER, Primary Examiner.

MILTON O. HIRSHFIELD, Examiner.

8. AN ACCELEROMETER COMPRISING A HOUSING, A CLOSED END ON SAID HOUSING,AN ACCELERATION DETECTING TRANSDUCER INCLUDING A BASE WITHIN SAIDHOUSING, COOPERATING MEANS ING SAID TRANSDUCER IN PREDETERMINEDALIGNMENT WITH SAID HOUSING, AND MECHANICAL MEANS EXTENDING THROUGH SAID